Unraveling sulfur chemistry in interstellar carbon oxide ices

Formyl (HCO•) and hydroxycarbonyl (HOCO•) radicals are key building blocks for complex organic molecule (COM) formation in the interstellar medium. The authors uncover efficient pathways to generate HCO• and HOCO• from primordial carbon oxides (CO and CO₂) via hydrogen atom transfer (HAT) reactions initiated by photochemistry of the hydroxidooxidosulfur radical (HOSO•), an intermediate in SO₂ photochemistry, in interstellar-analog CO and CO₂ ices at 16 K. Upon 266 nm photolysis of HOSO• embedded in CO ice, hydrogen-bonded complexes HCO…SO₂ and HOCO…SO form; the latter undergoes secondary HAT to produce CO₂HOS under irradiation. In CO₂ ice, photo-induced HAT yields HOCOSO₂. These HAT processes generate reactive acyl radicals within astronomical CO and CO₂ ices and help explain the coupling of sulfur and carbon ice-grain chemistry in cold molecular clouds and planetary atmospheres.

Xiaolong Li, Bo Lu, Lina Wang, Junfei Xue, Bifeng Zhu, Tarek Trabelsi, Joseph S. Francisco, Xiaoqing Zeng